Many people use mobile stations (MSs), such as cell phones and personal digital assistants (PDAs), to communicate with cellular wireless networks. These MSs and networks typically communicate with each other over a radio-frequency (RF) air interface according to a wireless communication protocol such as Code Division Multiple Access (CDMA), perhaps in conformance with one or more industry specifications such as IS-95 and IS-2000. Wireless networks that operate according to these specifications are also referred to as “1xRTT (1x) networks,” which stands for “Single Carrier Radio Transmission Technology”. These networks (referred to herein as “CDMA networks”) typically provide communication services such as voice, Short Message Service (SMS) messaging, and packet-data communication.
Typical CDMA networks include a plurality of base stations, each of which provide one or more wireless coverage areas, such as cells and sectors. As a general matter, sectors are used herein as examples of wireless coverage areas. When an MS is positioned in a sector, it can communicate over the RF air interface with the base station, and, in turn, over one or more circuit-switched and/or packet-switched signaling and/or transport networks to which the base station provides access. The base station and the MS conduct these communications over a frequency known as a carrier. Note that base stations may provide service in a sector on one carrier, or on more than one, and further that a given instance of a given carrier in a given sector may be referred to herein as a “sector/carrier.”
In general, a pilot channel functions to alert MSs in a given sector of the presence of a service-providing base station. Typically, the pilot channel conveys a value known as a pseudorandom number (PN) offset, which identifies the sector; in particular, by being offset from CDMA system time by a certain amount of time, the pilot channel conveys the PN offset. MSs generally use the pilot channel to coherently detect and demodulate the signal on a sector/carrier.
In a typical wireless network, an MS maintains different base-station-signal sets that typically include an active set, a candidate set, a neighbor set, and a remaining set. At a given time, the active set comprises one or more “active” sectors or coverage areas that are being used to maintain a call for the MS. The MS receives substantially the same information from each of the coverage areas in the active set and, on a frame-by-frame basis, selects the best signal to use. The candidate set comprises sectors that are not presently assigned to a call, but are strong enough to be used for a call. The neighbor set comprises sectors having signals that could be received with sufficient strength to enable successful communication. The remaining set comprises sectors the MS is in communication with that are not in the active, candidate or neighbor sets. The MS, one or more base stations, and/or a base station controller maintains in memory a list of the coverage areas in the MS's base-station-signal sets.
In existing systems, to facilitate a determination of which sectors should be in the MS's active and neighbor set, all base stations emit their pilot signal at a power level that is typically higher than other forward-link signals. An MS constantly measures the strength of each pilot that it receives, and notifies at least one base station when pilot strength for various coverage areas falls above or below designated thresholds. The base station, may, in turn, provide the MS with updated lists of the sectors in the various base-station-signal sets corresponding to that MS.
In a CDMA system configured as above, a link from the base station to the MS is called a forward link and a link from the MS to the base station is called a reverse link. All forward links from a given base station have the same PN offset. As stated, the PN offset is transmitted via the pilot signal, as one of the forward-link channels. An MS continuously monitors the pilot signal in order to obtain a good quality signal on the forward link. An MS located within the coverage area of the base station uses the pilot signal for synchronization. An MS can acquire the timing of the CDMA forward-link traffic channel from the pilot signal and obtain the phase reference for coherent demodulation.